Returns the SourceGutter object associated with windowType for view. Only TextWindowLeft
and TextWindowRight are supported, respectively corresponding to the left and right
gutter. The line numbers and mark category icons are rendered in the gutter corresponding to
TextWindowLeft.

The moveLines signal is a keybinding which gets emitted when the user initiates moving a
line. The default binding key is Alt+Up/Down arrow. And moves the currently selected lines, or the
current line by count. For the moment, only count of -1 or 1 is valid.